U.S. patent application number 14/862294 was filed with the patent office on 2016-04-07 for positive electrode having enhanced conductivity and secondary battery including the same.
This patent application is currently assigned to LG CHEM, LTD.. The applicant listed for this patent is LG CHEM, LTD.. Invention is credited to In Chul Kim, Eun Ju Lee, Sei Woon Oh, JungAh Shim.
Application Number | 20160099471 14/862294 |
Document ID | / |
Family ID | 55633453 |
Filed Date | 2016-04-07 |
United States Patent
Application |
20160099471 |
Kind Code |
A1 |
Oh; Sei Woon ; et
al. |
April 7, 2016 |
POSITIVE ELECTRODE HAVING ENHANCED CONDUCTIVITY AND SECONDARY
BATTERY INCLUDING THE SAME
Abstract
Disclosed is a positive electrode for secondary batteries
including a positive electrode mix coated on a current collector.
More particularly, disclosed are a positive electrode for secondary
batteries including a positive electrode mix coated on a current
collector and a secondary battery including the same, wherein the
current collector includes carbon nanotubes (CNTs) vertically grown
from a surface of the current collector, the positive electrode mix
contact the current collector in a state that at least a portion of
the positive electrode mix is interposed in a space between the
carbon nanotubes, and the positive electrode has high conductivity
and safety.
Inventors: |
Oh; Sei Woon; (Daejeon,
KR) ; Lee; Eun Ju; (Daejeon, KR) ; Kim; In
Chul; (Daejeon, KR) ; Shim; JungAh; (Daejeon,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LG CHEM, LTD. |
Seoul |
|
KR |
|
|
Assignee: |
LG CHEM, LTD.
Seoul
KR
|
Family ID: |
55633453 |
Appl. No.: |
14/862294 |
Filed: |
September 23, 2015 |
Current U.S.
Class: |
429/223 ;
427/122; 429/224; 429/231.3; 429/245 |
Current CPC
Class: |
H01M 4/0428 20130101;
H01M 4/663 20130101; H01M 2004/028 20130101; H01M 4/131 20130101;
H01M 4/505 20130101; Y02E 60/10 20130101; H01M 4/1391 20130101;
H01M 2004/021 20130101; H01M 4/525 20130101; H01M 2220/30 20130101;
H01M 4/0404 20130101; H01M 4/0416 20130101; H01M 2220/20
20130101 |
International
Class: |
H01M 4/66 20060101
H01M004/66; H01M 4/04 20060101 H01M004/04; H01M 4/505 20060101
H01M004/505; H01M 4/131 20060101 H01M004/131; H01M 4/525 20060101
H01M004/525 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 2, 2014 |
KR |
10-2014-0133017 |
Claims
1. A positive electrode for secondary batteries, comprising a
positive electrode mix comprising a positive electrode active
material coated on a current collector, wherein the current
collector comprises carbon nanotubes (CNTs) vertically grown from a
surface of the current collector, and the positive electrode mix
contacts the current collector in a state that at least a portion
of the positive electrode mix is interposed in a space between the
carbon nanotubes.
2. The positive electrode according to claim 1, wherein the
positive electrode mix comprises a conductive material.
3. The positive electrode according to claim 2, wherein the
conductive material is comprised in an amount of 0.1 to 10% by
weight based on a total weight of the positive electrode mix.
4. The positive electrode according to claim 1, wherein the
positive electrode mix does not comprise a conductive material.
5. The positive electrode according to claim 1, wherein the
positive electrode active material is a lithium transition metal
oxide comprising at least one selected from the group consisting of
nickel (Ni), cobalt (Co) and manganese (Mn).
6. The positive electrode according to claim 1, wherein the carbon
nanotubes have an average vertical growth length of 1 to 200
.mu.m.
7. The positive electrode according to claim 1, wherein the carbon
nanotubes have an average vertical growth length of 5 to 150
.mu.m.
8. The positive electrode according to claim 1, wherein the carbon
nanotubes have an average diameter of 0.4 to 20 nm.
9. The positive electrode according to claim 1, wherein the carbon
nanotubes have an average diameter of 10 to 20 nm.
10. The positive electrode according to claim 1, wherein the carbon
nanotubes are comprised in a mass of 0.1 to 10% based on a total
mass of the positive electrode mix.
11. The positive electrode according to claim 1, wherein the carbon
nanotubes are formed at an interval of 0.1 to 100 .mu.m.
12. The positive electrode according to claim 11, wherein the
carbon nanotubes are formed at a constant interval.
13. The positive electrode according to claim 11, wherein a
positive electrode mix coating layer is formed by coating a slurry
for the positive electrode mix after vertically growing carbon
nanotubes on a surface of the current collector.
14. The positive electrode according to claim 11, wherein the
positive electrode mix forms a coating layer in a state that the
carbon nanotubes are embedded therein.
15. The positive electrode according to claim 11, wherein the
positive electrode mix forms a coating layer to a vertically grown
height of the carbon nanotubes.
16. A method of manufacturing the positive electrode according to
claim 1, the method comprising: vertically growing carbon nanotubes
on a surface of a current collector; preparing a slurry for a
positive electrode mix; and coating the slurry for the positive
electrode mix on the current collector, in which the carbon
nanotubes are vertically grown, and then drying the same.
17. The method according to claim 16, wherein the vertically
growing is carried out by forming catalyst concentration areas, at
an interval of 0.1 to 100 .mu.m, in the current collector and
vertically growing the carbon nanotubes in each of the catalyst
concentration areas.
18. The method according to claim 17, wherein an interval between
the catalyst concentration areas is constant.
19. A lithium secondary battery comprising the positive electrode
according to claim 1.
20. A battery pack comprising the lithium secondary battery
according to claim 19.
21. A device comprising the battery pack according to claim 20.
22. The device according to claim 21, wherein the device is a
computer, a mobile phone, a wearable electronic device, a power
tool, an electric vehicle (EV), a hybrid electric vehicle, a
plug-in hybrid electric vehicle, an electric two-wheeled vehicle,
an electric golf cart or a system for storing power.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority from Korean
Application No. 10-2014-0133017 filed Oct. 2, 2014, the disclosure
of which is hereby incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a positive electrode for
secondary batteries including a positive electrode mix coated on a
current collector. More particularly, the present invention relates
to a positive electrode for secondary batteries wherein CNTs are
vertically grown in a surface of a current collector and at least a
portion of the positive electrode mix is disposed between the
CNTs.
BACKGROUND ART
[0003] In line with rapid increase in use of fossil fuels, demand
for alternative energy or clean energy is increasing. Thus, the
field of power generation and electrochemical electricity storage
is most actively studied.
[0004] As a representative example of electrochemical devices using
electrochemical energy, secondary batteries are currently used and
use thereof is gradually expanding.
[0005] Recently, as technical development and demand for portable
devices such as smartphones, notebooks, wearable devices, cameras,
etc. increase, demand for secondary batteries as energy sources is
rapidly increasing. Among secondary batteries, research on lithium
secondary batteries, which exhibit high energy density and
operation potential and have long cycle life and low self-discharge
rate, is underway and such lithium secondary batteries are
commercially available and widely used.
[0006] In addition, as interest in environmental problems is
increasing, research into electric vehicles, hybrid electric
vehicles, and the like that can replace vehicles using fossil
fuels, such as gasoline vehicles, diesel vehicles, and the like,
which are one of the main causes behind air pollution, is actively
underway. As a power source of electric vehicles, hybrid electric
vehicles, and the like, a secondary battery is used.
[0007] Such a lithium secondary battery has a structure wherein an
electrode assembly composed of a positive electrode including a
lithium transition metal oxide as an active material, a negative
electrode including a carbon-based active material and a separator
is impregnated with a lithium electrolyte. The positive electrode
and the negative electrode are manufactured by coating an electrode
mix on a current collector, and the electrode mix is prepared by
mixing an electrode mix composed of an electrode active material
for storing energy, a conductive material for providing electric
conductivity and a binder for adhering to electrode foil, and a
dispersion medium such as water, NMP, etc.
[0008] In general, a conductive material is added to an electrode
mix to enhance electric conductivity of an active material. In
particular, since a lithium transition metal oxide used as a
positive electrode active material has low electric conductivity,
adding a conductive material to a positive electrode mix is
essential.
[0009] Such a conductive material is added in an amount of about 3
to about 15% by weight based on the weight of a positive electrode
mix. When the conductive material is used in too small an amount,
interior resistance of a positive electrode increases and thus
battery performance is decreased. On the other hand, when the
conductive material is used in too large an amount, the content of
binder should be increased and thus the content of active material
is decreased, thereby leading to battery capacity reduction,
dispersibility decrease, etc.
[0010] Meanwhile, when a current collector foil is coated with a
positive electrode mix, a foil surface becomes smooth and thus
adhesion to an active material and conductivity are not secured,
whereby it is not easy to secure stable conductivity. In addition,
when a positive electrode mix is thickly coated to secure higher
capacity, it is difficult to evenly disperse a conductive material
composed of small particles, whereby uniform conductivity is not
guaranteed and slurry properties are deteriorated.
[0011] Accordingly, there is an urgent need for technology to
address such problems.
DISCLOSURE
Technical Problem
[0012] Therefore, the present invention has been made to solve the
above and other technical problems that have yet to be
resolved.
[0013] As a result of a variety of extensive and intensive studies
and experiments, the inventors of the present invention confirmed
that, when carbon nanotubes (CNTs) are vertically grown in a
surface of a current collector and a positive electrode mix
contacts a current collector in a state that at least a portion of
the positive electrode mix is interposed in a space between carbon
nanotubes as described below, adhesion between a positive electrode
active material and the current collector is enhanced and an entire
positive electrode mix coating layer has uniform and high
conductivity. Accordingly, capacity characteristics and rate
performance are enhanced and the content of a conductive material
included in the mixture is decreased, and thus, slurry
characteristics may be secured, thereby completing the present
invention.
Technical Solution
[0014] Accordingly, a positive electrode for secondary batteries
according to the present invention includes a positive electrode
mix including a positive electrode active material coated on a
current collector, wherein the current collector includes carbon
nanotubes vertically grown from a surface of the current collector,
and the positive electrode mix contacts the current collector in a
state that at least a portion of the positive electrode mix is
interposed in a space between the carbon nanotubes.
[0015] The positive electrode for secondary batteries according to
the present invention has a structure wherein the positive
electrode mix is interposed in a space between the carbon nanotubes
grown vertically in a metallic current collector at a predetermined
interval. Accordingly, swelling is induced only in a thickness
direction of the positive electrode, and, compared to a smooth
current collector, adhesion between the active material and the
current collector is enhanced.
[0016] In addition, even when the positive electrode mix disposed
between the vertically grown carbon nanotubes includes a small
amount of conductive material, conductivity may be secured through
a network with the entirely distributed carbon nanotubes.
Accordingly, it is easy to secure dispersibility of a positive
electrode mix slurry, and strong bonding force and fixation force
are increased by increasing contact areas to the carbon nanotubes
grown in the positive electrode mix and the current collector.
[0017] In addition, carbon nanotubes arranged in the same direction
facilitate migration of lithium ions, compared to irregularly
arranged carbon nanotubes, and thus, a solid electrolyte interphase
(SEI) layer is stably formed and reversible capacity is secured,
thereby enhancing capacity characteristics and rate
performance.
[0018] That is, the positive electrode for secondary batteries
according to the present invention includes the carbon nanotubes,
which are vertically grown from the current collector, and the
positive electrode mix, at least a portion of which is disposed
between the carbon nanotubes, thereby having superior conductivity,
capacity and rate performance. Hereinafter, a composition of the
positive electrode for secondary batteries according to the present
invention is described in more detail.
[0019] In the current collector, electrons migrate through
electrochemical reaction of an active material, and the current
collector is generally manufactured to a thickness of 3 to 500
.mu.m. The positive electrode current collector is not particularly
limited so long as it does not cause chemical changes in the
fabricated secondary battery and has high conductivity. For
example, the positive electrode current collector may be made of
stainless steel, aluminum (Al), nickel (Ni), titanium (Ti),
sintered carbon, or aluminum or stainless steel surface-treated
with carbon, nickel, titanium, silver, or the like.
[0020] The positive electrode current collector may have fine
irregularities at a surface thereof to increase bonding force to an
active material. The positive electrode current collector may be
used in any of various forms including films, sheets, foils, nets,
porous structures, foams, and non-woven fabrics, but the present
invention is not limited thereto.
[0021] These current collectors have minute irregularities at a
surface thereof and thus may increase bonding force to an active
material. The current collectors may be used in any of various
forms including films, sheets, foils, nets, porous structures,
foams, and non-woven fabrics, but the present invention is not
limited thereto.
[0022] Next, the carbon nanotubes vertically grown in the current
collector have a shape wherein one or more graphite layers are
rolled, and have high conductivity and a wide surface area. Carbon
nanotubes have both zigzag and armchair shapes. These shapes are
dependent upon an arrangement state of a hexagonal ring structure
when a graphite layer is rolled into a concentric circle with
respect to an axis of a nanotube. That is, properties and shapes of
the carbon nanotubes depend upon an angle when wound into a spiral
shape with respect to a coaxial shaft of a tube. A carbon nanotube
has an armchair shape when there is a hexagonal diagonal vector
constituting a graphite layer upon being wound into a concentric
circle, and a carbon nanotube has a zigzag shape when there is a
vector in a vertical direction to the hexagonal side upon being
wound into a concentric circle.
[0023] Such carbon nanotubes may have an average vertical growth
length of 1 to 200 .mu.m, particularly 5 to 150 .mu.m. Outside the
range, when the average vertical growth length is greater than 200
.mu.m, the thickness of the positive electrode mix increases, and a
resultant positive electrode mix, the thickness of which is
increased, has decreased electrolyte wettability. When the carbon
nanotubes have an average vertical growth length of less than 1
.mu.m, it is difficult to secure desired conductivity. In addition,
the thickness of the positive electrode mix is decreased depending
upon the length of the carbon nanotubes, and thus, battery capacity
is decreased.
[0024] In addition, the carbon nanotubes may have an average
diameter of 0.4 to 20 nm. In particular, the carbon nanotubes may
have a single wall structure or a multi-wall structure. When the
carbon nanotubes have a single wall structure, an average diameter
thereof is generally 0.4 to 2 nm. When the carbon nanotubes have a
multi-wall structure, an average diameter thereof is generally 10
to 20 nm.
[0025] Conductivity of the carbon nanotubes is intimately related
to chirality of each tube. Since single-walled carbon nanotubes
(SWCNTs) have single-chirality, conductivity may be easily
controlled. Since multi-walled carbon nanotubes (MWCNTs) have
multi-chirality, it is difficult to control conductivity, but it is
possible to insert lithium ions between graphene layers and to
intercalate lithium ions. Accordingly, a layer structure becomes
stable, and an SEI film may be easily formed due to surface area
increase by a multi-layer structure, thus being more preferably
used in lithium secondary batteries.
[0026] Accordingly, the average diameter of the carbon nanotubes is
preferably 0.4 to 20 nm. In regard to multi-wall formation, the
average diameter of the carbon nanotubes is more preferably 10 to
20 nm Outside this range, when an average diameter of the carbon
nanotubes is less than 0.4 nm, it is difficult to secure desired
conductivity and formation of SWCNTs may be difficult. When an
average diameter of the carbon nanotubes is greater than 20 nm,
time and costs substantially increase.
[0027] In addition, the carbon nanotubes may be included in an
amount of 0.1 to 10% based on the total weight of the positive
electrode mix. Outside this range, when the mass of nanotubes grown
in the current collector is less than 0.1%, it is difficult to
obtain desired effects. On the other hand, when nanotubes are
included in a mass of greater than 10%, the amount of active
material is relatively decreased and capacity may be decreased due
to inclusion of a conductive material unnecessary in capacity
expression. For the same reason, the carbon nanotubes may be
included in an amount of 0.3 to 9%, more particularly 0.5 to 7%
based on the total weight of the positive electrode mix.
[0028] Next, a positive electrode mix generally denotes a solid
content remaining after preparing a slurry by dispersing a positive
electrode active material, a binder and a conductive material in a
dispersion medium and then volatilizing the dispersion medium by
coating and drying the slurry on a current collector.
[0029] The positive electrode for secondary batteries according to
the present invention may or might not include a conductive
material. That is, a conductive material of the positive electrode
mix may be substituted with the carbon nanotubes vertically grown
in the current collector. Alternatively, a small amount of a
conductive material may be included in the positive electrode mix,
thereby exhibiting excellent conductivity through interaction with
the vertically grown carbon nanotube.
[0030] In general, a conductive material has low miscibility to a
dispersion medium and thus it is difficult to disperse and diffuse
solids. Accordingly, a slurry including a large amount of
conductive material is not uniform, which deteriorates battery
properties. Such a phenomenon is further intensified by
sedimentation occurring when an evaporation amount of a dispersion
medium increases or waiting time for slurry-coating is
extended.
[0031] Accordingly, when the content of conductive material is
increased to manufacture a high-capacity battery, processability is
deteriorated. The positive electrode for secondary batteries
according to the present invention may secure a sufficient
conductivity using a small amount of conductive material in a
mixture even when a relatively wide conductivity range is secured,
thereby enhancing processability. In addition, due to interaction
between the carbon nanotubes vertically grown in the current
collector and the conductive material included in a slurry,
conductivity may be stably secured without a congested area even
when the mixture coating layer is thick.
[0032] Such a conductive material is an ingredient for further
enhancing conductivity of an active material. When a conductive
material is included in a positive electrode mix, the positive
electrode mix may be included in an amount of 0.1% by weight to 10%
by weight, in particular 0.1% by weight to 5% by weight, more
particularly 0.1% by weight to 4% by weight based on the total
weight of the positive electrode mix. Even when the conductive
material is included in a relatively small amount, predetermined
conductivity may be secured through interaction with the carbon
nanotube vertically grown in the current collector.
[0033] There is no particular limit as to the conductive material,
so long as it does not cause chemical changes in the fabricated
battery and has conductivity. Examples of conductive materials
include graphite such as natural or artificial graphite; carbon
black such as carbon black, acetylene black, Ketjen black, channel
black, furnace black, lamp black and thermal black; carbon
derivatives such as carbon nanotubes and fullerenes; conductive
fibers such as carbon fibers and metallic fibers; metallic powders
such as carbon fluoride powder, aluminum powder and nickel powder;
conductive whiskers such as zinc oxide and potassium titanate;
conductive metal oxides such as titanium oxide; and polyphenylene
derivatives.
[0034] The binder is a component assisting in binding between the
active material and the conductive material and in binding of the
electrode active material to the current collector. The binder is
typically added in an amount of 1 to 50 wt % with respect to the
total weight of the mixture including the positive electrode active
material. Examples of the binder include polyvinylidene fluoride,
polyvinyl alcohols, carboxymethylcellulose (CMC), starch,
hydroxypropylcellulose, regenerated cellulose, polyvinyl
pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,
ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM,
styrene-butadiene rubber, fluorine rubber, and various
copolymers.
[0035] Meanwhile, the positive electrode active material is not
specifically limited so long as lithium ions may be intercalated
and deintercalated. In particular, the positive electrode active
material may be a lithium transition metal oxide including at least
one selected from the group consisting of nickel (Ni), cobalt (Co)
and manganese (Mn).
[0036] More particularly, lithium transition metal oxide includes
two or more transition metals and, for example, may be a layered
compound such as a lithium cobalt oxide (LiCoO.sub.2), lithium
nickel oxide (LiNiO.sub.2) and the like substituted with one or
more transition metals; lithium manganese oxide substituted with
one or more transition metals; lithium nickel-based oxide
represented by formula, LiNi.sub.1-yM.sub.yO.sub.2 (where M
includes at least one of Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn and Ga,
and 0.01.ltoreq.y.ltoreq.0.7); and a lithium nickel cobalt
manganese complex oxide represented by
Li.sub.1+zNi.sub.bMn.sub.cCo.sub.1-(b+c+d)M.sub.dO.sub.(2-e)A.sub.e
(where -0.5.ltoreq.z.ltoreq.0.5, 0.1.ltoreq.b.ltoreq.0.8,
0.1.ltoreq.c.ltoreq.0.8, 0.ltoreq.d.ltoreq.0.2,
0.ltoreq.e.ltoreq.0.2 and b+c+d<1, M is Al, Mg, Cr, Ti, Si or Y,
and A is F, P or Cl) such as
Li.sub.1+zNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2,
Li.sub.1+zNi.sub.0.4Mn.sub.0.4Co.sub.0.2O.sub.2 and the like.
[0037] The above lithium transition metal oxides used as a positive
electrode active material have low electric conductivity and thus a
conductive material is generally added thereto. Such a conductive
material has a minute particle size and thus dispersibility of a
slurry is deteriorated, thereby deteriorating processability.
[0038] In addition, a positive electrode manufactured from a slurry
having non-uniform dispersibility as described above leads to
physical or electrochemical problems. Accordingly, when a secondary
battery is manufactured using the positive electrode, capacity
characteristics, rate characteristics, and overall battery
properties may be deteriorated.
[0039] In the positive electrode for secondary batteries according
to the present invention, carbon nanotubes are vertically grown at
a predetermined interval in a current collector, and the positive
electrode mix is interposed in a space between carbon nanotubes.
Accordingly, the content of conductive material in a slurry is
reduced and, at the same time, a positive electrode manufactured
using the slurry has sufficient conductivity. In addition,
processability in slurry production is enhanced.
[0040] The positive electrode mix may further include at least one
material selected from the group consisting of a viscosity control
agent and a filler, other than a positive electrode active
material, a conductive material and a binder.
[0041] The viscosity control agent is an ingredient for controlling
the viscosity of a positive electrode mix so as to facilitate a
mixing process of a positive electrode mix and a process of
spreading the same over a collector and may be added in an amount
of maximally 30% by weight based on the total weight of the
positive electrode mix. Examples of such a viscosity control agent
include carboxymethylcellulose, polyacrylic acids, etc., but the
present invention is not limited thereto.
[0042] The filler is used as an aid to inhibit positive electrode
expansion and is not particularly limited so long as it is a
fibrous material that does not cause chemical changes in the
fabricated secondary battery. Examples of the filler include
olefin-based polymers such as polyethylene and polypropylene; and
fibrous materials such as glass fiber and carbon fiber.
[0043] Meanwhile, the carbon nanotubes are preferably formed at an
interval of 0.1 to 100 .mu.m. In addition, the carbon nanotubes may
be formed at a constant interval.
[0044] Outside the range, when an interval between the carbon
nanotubes is greater than 100 .mu.m, it is difficult to form a
minute electron migration network. When an interval between the
carbon nanotubes is less than 0.1 .mu.m, it is difficult to
introduce a positive electrode mix slurry into the vertically grown
carbon nanotubes. For the same reason, an interval between the
carbon nanotubes may be 1 .mu.m to 80 .mu.m, more particularly 5
.mu.m to 60 .mu.m.
[0045] That is, the positive electrode for secondary batteries
according to the present invention is manufactured by forming a
positive electrode mix coating layer through coating of a slurry
for the positive electrode mix after vertically growing carbon
nanotubes on a surface of the current collector. Here, an interval
between the carbon nanotubes may be controlled to secure high
conductivity within a range within which the positive electrode mix
slurry is introduced between the carbon nanotubes.
[0046] Hereinafter, a method of forming an internal between the
carbon nanotubes and a method of vertically growing are described
in more detail.
[0047] Meanwhile, the positive electrode mix may form a coating
layer in a state that the carbon nanotubes are embedded therein. In
addition, the positive electrode mix may form a coating layer to a
vertically grown height of the carbon nanotubes.
[0048] When a coating layer wherein the thickness of the coating
layer is smaller than a vertical length of the carbon nanotube is
formed, an electrode surface may be non-uniform. When a coating
layer wherein the thickness of the coating layer is larger than a
vertical length of the carbon nanotube is formed, conductivity may
be decreased in a portion in which the carbon nanotubes are not
present.
[0049] In addition, the present invention provides a method of
manufacturing a positive electrode for secondary batteries
including a positive electrode mix that includes a positive
electrode active material on a current collector, wherein the
current collector includes carbon nanotubes vertically grown from a
surface thereof, and the positive electrode mix contacts the
current collector in a state that at least a portion of the
positive electrode mix is interposed in a space between the carbon
nanotubes.
[0050] In particular, the manufacturing method includes:
[0051] (a) vertically growing carbon nanotubes on a surface of a
current collector;
[0052] (b) preparing a slurry for a positive electrode mix; and
[0053] (c) coating the slurry for a positive electrode mix on the
current collector, in which the carbon nanotubes are vertically
grown, and then drying the same.
[0054] In order to more particularly describe the manufacturing
method, a method of manufacturing a positive electrode for
secondary batteries is schematically illustrated in FIG. 1.
[0055] Referring to FIG. 1, in the vertically growing (a), carbon
nanotubes are vertically grown in a surface of a current collector,
and the carbon nanotubes may be vertically grown in a current
collector using a carbon source including a catalyst and aromatic
hydrocarbon under a condition of high temperature of 800.degree. C.
to 1200.degree. C. and an inert gas atmosphere such as argon,
etc.
[0056] In particular, the vertically growing (a) may be carried out
by forming catalyst concentration areas, at an interval of 0.1 to
100 .mu.m, in the current collector and vertically growing the
carbon nanotubes in each of the catalyst concentration areas.
[0057] More particularly, the vertical growth process may be
carried out through a CVD method, a PECVD method, or the like. In
order to grow the carbon nanotubes separated by a predetermined
interval, catalyst concentration areas separated by a predetermined
interval are formed in a current collector using pre-heating or a
laser, before growing the carbon nanotubes.
[0058] Here, an interval between the catalyst concentration areas
may be constant.
[0059] In such catalyst concentration areas, carbon nanotubes grow,
and one carbon nanotube or multiple carbon nanotubes may be formed
per area. The size and density of the areas may be controlled with
a zeolite catalyst, etc. considering the thickness of the current
collector.
[0060] In the preparing (b), a positive electrode slurry
constituting the positive electrode mix is prepared. The positive
electrode slurry is prepared by mixing a positive electrode active
material, a conductive material, a binder, etc. with a dispersion
medium. As described above, as the positive electrode active
material, a lithium transition metal oxide including at least one
selected from the group consisting of nickel, cobalt, and manganese
may be used. The slurry constituting the positive electrode mix
does not include a conductive material or includes a small amount
of conductive material, thereby alleviating property deterioration
of the slurry.
[0061] The dispersion medium is not specifically limited and,
preferably, a material, a polymer particle shape of which may be
maintained when the positive electrode slurry according to the
present invention is coated and dried on a current collector and
which is a liquid at normal temperature and pressure, is
preferable. Examples of the dispersion medium, but do not limited
to, include water; alcohols such as methanol, ethanol, propanol,
isopropanol, butanol, isobutanol, s-butanol, t-butanol, pentanol,
isopentanol, hexanol, etc.; ketones such as acetone,
methylethylketone, methylpropylketone, ethylpropylketone,
cyclopentanone, cyclohexanone, cycloheptanone, etc.; ethers such as
methylethylether, diethylether, dipropylether, diisopropylether,
dibutylether, diisobutylether, di-n-amylether, diisoamylether,
methylpropylether, methylisopropylether, methylbutylether,
ethylpropylether, ethylisobutylether, ethyl-n-amylether,
ethylisoamylether, tetrahydrofuran, etc.; lactones such as
gamma-butyrolactone, delta-butyrolactone, etc.; lactam such as
beta-lactam, etc.; and electrolyte dispersion media described
below. The dispersion medium may be a mixture of two or more
media.
[0062] As such, a slurry prepared through (b) the preparing is used
in a positive electrode for secondary batteries by being coated and
then dried on the current collector in which the carbon nanotubes
are vertically grown according to (c) the coating.
[0063] The manufactured positive electrode for secondary batteries
has an increased contact area to a current collector and an active
material, and thus, adhesion is enhanced and conductivity may be
efficiently secured. In addition, as illustrated in FIG. 1, the
vertically grown carbon nanotubes may control expansion direction
of the positive electrode mix.
[0064] That is, an expansion direction of an electrode is generally
vertical, but an electrode may expand in any direction according to
a charge/discharge process. Here, the shape and volume of each
electrode included in a battery may be somewhat different. However,
since, in the positive electrode for secondary batteries according
to the present invention, the CNTs vertically grown in the current
collector guide an expansion direction, predictability of the
expansion direction may be increased.
[0065] In addition, the present invention provides a lithium
secondary battery including the positive electrode for secondary
batteries. The lithium secondary battery includes, along with the
positive electrode, a negative electrode, a lithium-containing
non-aqueous-based electrolyte which functions as a medium through
which lithium ions migrate between the negative electrode and the
positive electrode, and a separator.
[0066] The negative electrode is manufactured by coating a negative
electrode mix including a negative electrode active material on a
current collector in a manner similar to the positive electrode.
The negative electrode active material may be, for example, carbon
and graphite materials such as natural graphite, artificial
graphite, expandable graphite, carbon fiber, hard carbon, carbon
black, carbon nanotubes, fullerenes, and activated carbon; metals
alloyable with lithium such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge,
Pb, Pd, Pt, Ti, and the like and compounds including these
elements; complexes of metals and compounds thereof and complexes
of carbon and graphite materials; lithium-containing nitrides; or
the like.
[0067] The lithium-containing non-aqueous electrolyte is composed
of a non-aqueous electrolyte and a lithium salt.
[0068] Examples of the non-aqueous electrolyte include aprotic
organic dispersion media such as N-methyl-2-pyrrolidone,
N-methyl-2-pyrrolidone carbonate, ethylene carbonate, butylene
carbonate, dimethyl carbonate, diethyl carbonate,
gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydrofuran,
2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane,
formamide, dimethylformamide, dioxolane, acetonitrile,
nitromethane, methyl formate, methyl acetate, phosphoric acid
triester, trimethoxy methane, dioxolane derivatives, sulfolane,
methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene
carbonate derivatives, tetrahydrofuran derivatives, ether, methyl
propionate, and ethyl propionate.
[0069] The lithium salt is a material that is readily soluble in
the non-aqueous electrolyte and examples thereof include, but are
not limited to, LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4,
LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3,
LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4,
CH.sub.3SO.sub.3Li, (CF.sub.3SO.sub.2).sub.2NLi, chloroborane
lithium, lower aliphatic carboxylic acid lithium, lithium
tetraphenyl borate, and imides.
[0070] As desired, an organic solid electrolyte, an inorganic solid
electrolyte and the like may be used.
[0071] Examples of the organic solid electrolyte include
polyethylene derivatives, polyethylene oxide derivatives,
polypropylene oxide derivatives, phosphoric acid ester polymers,
polyagitation lysine, polyester sulfide, polyvinyl alcohols,
polyvinylidene fluoride, and polymers containing ionic dissociation
groups.
[0072] Examples of the inorganic solid electrolyte include, but are
not limited to, nitrides, halides and sulfates of lithium (Li) such
as Li.sub.3N, LiI, Li.sub.5NI.sub.2, Li.sub.3N--LiI--LiOH,
LiSiO.sub.4, LiSiO.sub.4--LiI--LiOH, Li.sub.2SiS.sub.3,
Li.sub.4SiO.sub.4, Li.sub.4SiO.sub.4--LiI--LiOH, and
Li.sub.3PO.sub.4--Li.sub.2S--SiS.sub.2.
[0073] In addition, in order to improve charge/discharge
characteristics and flame retardancy, for example, pyridine,
triethylphosphite, triethanolamine, cyclic ether, ethylenediamine,
n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur,
quinone imine dyes, N-substituted oxazolidinone, N,N-substituted
imidazolidine, ethylene glycol dialkyl ether, ammonium salts,
pyrrole, 2-methoxy ethanol, aluminum trichloride or the like may be
added to the non-aqueous electrolyte. If necessary, in order to
impart incombustibility, the non-aqueous electrolyte may further
include halogen-containing dispersion media such as carbon
tetrachloride and ethylene trifluoride. Further, in order to
improve high-temperature storage characteristics, the non-aqueous
electrolyte may further include carbon dioxide gas, and
fluoro-ethylene carbonate (FEC), propene sultone (PRS) and the like
may be further included.
[0074] The separator is disposed between the positive electrode and
the negative electrode. An insulating thin film having high ion
permeability and mechanical strength is used as the separator. The
separator typically has a pore diameter of 0.01 to 10 .mu.m and a
thickness of 5 to 300 .mu.m. As the separator, sheets or non-woven
fabrics made of an olefin polymer such as polypropylene, glass
fibers or polyethylene, which have chemical resistance and
hydrophobicity, are used. When a solid electrolyte such as a
polymer is employed as the electrolyte, the solid electrolyte may
also serve as both the separator and electrolyte.
[0075] The secondary battery according to the present invention may
be used as a battery pack in a unit cell type. The battery pack may
also be used as a unit cell in medium/large battery modules used as
power sources of small devices and medium/large devices.
[0076] Particular embodiments of the device include, but are not
limited to, computers, mobile phones, wearable electronic devices,
power tools, electric vehicles (EVs), hybrid electric vehicles,
plug-in hybrid electric vehicles, electric two-wheeled vehicles,
electric golf carts, systems for storing power, etc.
[0077] The above devices or equipments are publicly known in the
art, and thus, detailed descriptions thereof are omitted.
Effects of Invention
[0078] As apparent from the fore-going, the positive electrode for
secondary batteries according to the present invention includes a
positive electrode mix coated on a current collector, wherein the
current collector includes carbon nanotubes vertically grown from a
surface thereof, and the positive electrode mix contacts a current
collector in a state that at least a portion of the positive
electrode mix is interposed in a space between the carbon
nanotubes. Accordingly, even when a small amount of conductive
material is included in the positive electrode mix, predetermined
conductivity is secured, and thus, safety of a slurry is increased
and carbon nanotubes arranged in the same direction secures
electron diffusivity in an electrode thickness direction, thereby
enhancing capacity and rate performance of a secondary battery.
BRIEF DESCRIPTION OF THE DRAWINGS
[0079] The above and other objects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0080] FIG. 1 is a schematic view illustrating a process of
manufacturing a positive electrode for secondary batteries
according to an embodiment of the present invention.
BEST MODE
[0081] Now, the present invention will be described in more detail
with reference to the following examples, comparative examples and
experimental examples. These examples are provided only for
illustration of the present invention and should not be construed
as limiting the scope and spirit of the present invention.
Example 1
Manufacture of Positive Electrode for Secondary Batteries
[0082] Catalyst concentration areas were formed by heating a 25
.mu.m aluminum current collector to 800.degree. C. in the presence
of Fe and Ni catalysts, and the current collector and a carbon
source were loaded together in a CVD furnace under an Ar/H.sub.2
atmosphere, followed by elevating from room-temperature to
750.degree. C. Subsequently, cooling to 250.degree. C. was carried
out, and thus, CNTs were vertically grown in the catalyst
concentration areas.
[0083] In a positive electrode mix slurry, N-methyl-2-pyrrolidone
(NMP) was used as a dispersion medium, and 96 parts by weight of
LiCoO.sub.2 as a positive electrode active material, 2.2 parts by
weight of Super-P as a conductive material and 1.5 parts by weight
of PVdF as a binder were used based on the 100 parts by weight of
the positive electrode mix (solid content).
[0084] The positive electrode mix slurry was coated and dried on a
current collector including CNTs vertically grown therein and then
pressed, thereby manufacturing a positive electrode for secondary
batteries. Here, the vertically grown CNTs were included in an
amount of 0.3 parts by weight based on the total weight of the
positive electrode mix.
Example 2
[0085] A positive electrode for secondary batteries was
manufactured using the same process and contents as in Example 1,
except that 2.0 parts by weight of Super-P as a conductive material
and 0.5 parts by weight of vertically grown CNTs were used based on
100 parts of a positive electrode mix slurry.
Comparative Example 1
[0086] In a positive electrode mix slurry, N-methyl-2-pyrrolidone
(NMP) was used as a dispersion medium, and 96 parts by weight of
LiCoO.sub.2 as a positive electrode active material, 2.2 parts by
weight of Super-P as a conductive material, 0.3 parts by weight of
general CNTs, 1.5 parts by weight of PVdF as a binder were mixed
based on 100 parts by weight of the positive electrode mix (solid
content).
[0087] The positive electrode mix slurry was coated, dried, and
then pressed on a current collector as in Example 1, except that,
in the current collector, the general current collector CNTs were
not vertically grown.
Comparative Example 2
[0088] A positive electrode for secondary batteries was
manufactured using the same process and contents as in Comparative
Example 1, except that 2.0 parts by weight of Super-P as a
conductive material and 0.5 parts by weight of general CNTs were
used based on 100 parts of a positive electrode mix slurry.
Comparative Example 3
[0089] A positive electrode for secondary batteries was
manufactured using the same process and contents as in Comparative
Example 1, except that 2.5 parts by weight of Super-P as a
conductive material and 0 parts by weight of general CNTs were used
based on 100 parts of a positive electrode mix slurry.
Comparative Example 4
[0090] A positive electrode for secondary batteries was
manufactured using the same process and contents as in Example 1,
except that 96.5 parts by weight of LiCoO.sub.2 as a positive
electrode active material, 2.5 parts by weight of Super-P as a
conductive material and 1 part by weight of PVDF were used.
Experimental Example 1
Conductivity Test
[0091] Penetration resistance of positive electrodes manufactured
according to Examples and Comparative Examples was measured.
Results are summarized in Table 1 below.
TABLE-US-00001 TABLE 1 Penetration resistance (.OMEGA.) Example 1
22.114 Example 2 18.186 Comparative 29.184 Example 1 Comparative
20.784 Example 2 Comparative 42.992 Example 3 Comparative 63.08
Example 4
[0092] Referring to Table 1, it can be confirmed that, although
Example 1, Comparative Example 1, Example 2 and Comparative Example
2 include the same amount of Super-P and CNTs based on the total
amount of the positive electrode, the examples that include
vertically grown CNTs exhibit low resistance compared to the
comparative examples that include general CNTs, and Examples 1 and
2 and Comparative Examples 1 and 2 including CNTs exhibit low
resistance, i.e., high conductivity compared to Comparative Example
3 and 4 that do not include CNTs.
[0093] In the case of the vertically grown CNTs, conductive
passages through which electrons penetrate in a thickness
direction, i.e., a vertical direction, of the electrode are formed
and thus uniform conductive passages are formed throughout the
electrode. Accordingly, high conductivity is exhibited when
compared to general CNTs in which conductive passages of electrons
are randomly formed.
[0094] Meanwhile, when Examples 1 and 2 are compared, it can be
confirmed that Example 2, in which a ratio of the vertical CNTs is
higher, has higher conductivity, and, since the amount of the
conductive material included in the positive electrode mix of
Example 2 is smaller than that in Example 1, processability may be
enhanced upon manufacturing a positive electrode according to
Example 2, compared to the case of Example 1.
Manufacture of Lithium Secondary Battery
Example 3
[0095] Water was used as a dispersion medium in a negative
electrode, and 98.3 parts by weight of a negative electrode active
material, 0.5 parts by weight of PVdF as a binder and 1.2 parts by
weight of CMC as a thickener were mixed based on 100 parts by
weight of a negative electrode mix to prepare a negative electrode
mix slurry. The prepared slurry was coated, dried and pressed on a
current collector, thereby manufacturing a negative electrode.
[0096] A surface of a positive electrode plate manufactured
according to Example 1 was punched into a size of 12.60 cm.sup.2,
and a surface of a negative electrode plate was punched into a size
of 13.33 cm.sup.2, thereby manufacturing a mono-cell. A tab was
attached to upper portions of the positive electrode and a negative
electrode, and a separator composed of a microporous polyolefin
membrane was disposed between a negative electrode and a positive
electrode. A resultant product was loaded in an aluminum pouch and
then 500 mg of an electrolyte was injected into the pouch. An
electrolyte was prepared by dissolving LiPF.sub.6 electrolyte to a
concentration of 1 M using a dispersion medium mixture that
included ethyl carbonate (EC), diethyl carbonate (DEC) and
ethyl-methyl carbonate (EMC) in a volume ratio of 4:3:3.
[0097] Subsequently, after sealing the pouch using a vacuum package
machine and standing the same at room temperature for 12 hours,
constant-current charging was performed in a ratio of about 0.05 C
and constant-voltage charging, in which the same voltage is
maintained until a current reached about 1/6 of initial current,
was performed. At this time, gas occurred within a cell and thus
degasification and resealing were performed, thereby manufacturing
a lithium secondary battery.
Comparative Example 5
[0098] A lithium secondary battery was manufactured in the same
manner as in Example 3, except that a positive electrode for
secondary batteries manufactured according to Comparative Example 1
was used.
Experimental Example 2
[0099] Using the lithium secondary batteries manufactured according
to Example 3 and Comparative Example 5, capacity thereof was
measured according to a constant-current charge/discharge method.
As a result, Example 3 and Comparative Example 5 exhibited similar
initial capacity, but a capacity conservation ratio of respectively
88% and 82% after 800 cycles upon charge/discharge of 1 C/1 C.
Accordingly, it can be confirmed that Example 3 exhibit excellent
high-current characteristics.
[0100] As described above, the positive electrode used in Example 3
has conductive passages penetrated in a vertical direction, thus
having increased resistance, compared to Comparative Example 5 in
which conductive passages are randomly formed. Accordingly, by
using the positive electrode according to the present invention
including the vertically grown CNTs, a secondary battery having
superior capacity and rate characteristics under high current may
be provided.
[0101] Although the preferred embodiments of the present invention
have been disclosed for illustrative purposes, those skilled in the
art will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *